The present invention relates generally to bulk acoustic wave resonators and filters. More particularly, the present invention relates to acoustically coupled thin-film bulk acoustic resonators (FBARS).
Thin-film bulk acoustic resonators (FBARS) are generally fabricated as a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When electrical signal (for example, radio frequency (RF) signal) is applied to the electrodes, mechanical wave is generated in the piezoelectric layer. Resonance of an FBAR occurs when the wavelength of the mechanical wave generated is approximately twice the thickness of its piezoelectric layer. The resonant frequency of an FBAR (thus, the RF signal exciting the FBAR) can range in the order of hundreds of MHz to many tens of GHz. FBARS are often used to filter electrical signals at these frequencies.
For a typical electrical signal filter application, multiple FBARS are used to form a band-pass filter where electrical signal having frequency within a predetermined and relatively narrow range (“band”) of frequencies is allowed to pass while electrical signal having frequency outside the band is blocked or redirected to, for example, ground. These FBARS can be electrically coupled or acoustically coupled to each other. To acoustically couple two FBARS, the FBARS are fabricated vertically with a decoupling layer between the FBARS.
FIG. 1 is a cut-away cross-sectional view of a prior art filter apparatus 100 having two FBAR pairs, each FBAR pair acoustically coupled. The filter 100 includes a first FBAR 110, a second FBAR 120, a third FBAR 130, and a fourth FBAR 140. The first pair includes the first FBAR 110 and the second FBAR 120 acoustically coupled to each other. The second pair includes the third FBAR 130 and the fourth FBAR 140 acoustically coupled to each other.
The first FBAR 110 includes a top electrode 112, a bottom electrode 114, and a portion of a first piezoelectric layer 116 that is sandwiched between the electrodes 112 and 114. The second FBAR 120 includes a portion of a top electrode layer 122 situated under the first FBAR 110, a portion of a bottom electrode layer 124 situated under the first FBAR 110, and a portion of a second piezoelectric layer 126 situated under the first FBAR 110. Lateral boundaries of the second FBAR 120 are defined by the overlaps between the electrode 122 and 124. The first FBAR 110 and the second FBAR 120 are acoustically coupled by a decoupling layer 102.
The fourth FBAR 140 includes a top electrode 142, a bottom electrode 144, and a portion of the first piezoelectric layer 116 that is sandwiched between the electrodes 142 and 144. The third FBAR 130 includes a portion of the top electrode layer 122 situated under the fourth FBAR 140, a portion of the bottom electrode layer 124 situated under the fourth FBAR 140, and a portion of the second piezoelectric layer 126 situated under the fourth FBAR 140. The third FBAR 130 and the fourth FBAR 140 are acoustically coupled by the decoupling layer 102. The second FBAR 120 and the third FBAR are electrically coupled via the common electrode layer.
Input electrical signal is injected to the electrodes 112 and 114 exciting the first FBAR 110 thus generating acoustic signal. The generated acoustic signal is acoustically coupled to the second FBAR 120. The degree of acoustic coupling is determined by the decoupling layer 102, typically implemented as multiple sub-layers. The second FBAR 120 converts the coupled portion of the acoustic signal to electrical signal. The electrical signal is electrically coupled to the third FBAR 130.
The coupled electrical signal excites the third FBAR 130 into generating acoustic signal. The generated acoustic signal is acoustically coupled to the fourth FBAR 140. Again, the degree of acoustic coupling is determined by the decoupling layer 102. The fourth FBAR 140 converts the coupled acoustic signal to electrical signal which is the output electrical signal of the filter 100. The output electrical signal includes only the desired portions (band) of the input electrical signal while undesired portions are blocked, grounded, or both.
Such filter apparatus can be found, for example, in FIG. 3 of U.S. Pat. No. 6,670,866 issued to Ellaet et al. on Dec. 30, 2003 and FIG. 4 of U.S. Pat. No. 6,720,844 issued to Lakin on Apr. 13, 2004.
As illustrated in FIG. 1 and the cited Figures of the cited prior art references, the filter apparatus 100 is often fabricated over an acoustic mirror 104, which, in turn, is fabricated above a surface 105 of a substrate 106. The prior art filter apparatus 100 suffers from a number of shortcoming. For example, as illustrated, the electrodes 112, 114, 142, and 144 (to which input and output signal connections are made) are relatively distal (vertically 109 in FIG. 1) from the top surface 105 of the substrate 106 on which connection pads, for example, a connection pad 108 exists. Reliable connections (connecting the electrodes 112, 114, 142, and 144 to such signal traces) are difficult to fabricate due, in part, to the vertical distance 109 such connections are required to span, and sharp corners 107 that such connection would need to include as illustrated by connector 117.
Accordingly, there remains a need for improved coupled acoustic resonators that overcome these shortcomings.